Heroin/fent/meth traffickers and dealers are not mostly white.....They are Mexican and black. So, the hard line Trump wants to take is just the same old "Law& Order" dog whistle that has been blaring from the right for years....Hence, Sessions rescinded the memo, and encouraged prosecutors to seek the highest sentence range possible, which targets low level participants in drug conspiracies, which are mostly minorities.
Mexicans primarily traffic meth, cocaine, heroin and fentanyl to places like Chicago/Baltimore/New York/Atlanta, where the work makes its way down to the bottom tier of the trade, which is rock cocaine and retail leevl heroin sales.
If you would like to see how these things work, read the DOJ press releases for your district, and watch for the "white" drug dealers, there aren't very many. In my city alone, last year, over 140 gang members were charged with federal crimes related to meth and heroin sales, there are barely any white people in the charging documents.